Of course you will not get dollar directly and the Steem rate could change a lot. The risk seem very low because even the price go down to 0.25$ per Steem you still get back more money than what you invested.
This is my main concern.... after I decide to Power Down, I have to hope that the value of Steem doesn't decrease significantly during the 2 years it takes for me to get my payment back.
But Steem is very new, and hopefully this is just the beginning of the uptrend in both user activity and price of Steem